Apparatus for monitoring suspended aerosols and particulates in a gas

ABSTRACT

Apparatus for monitoring suspended aerosols and solid particulates in a gas. The apparatus consists basically of a filter in a spring-mounted holder, means such as a pump for moving gas through the filter, means for causing the filter assembly to oscillate substantially in a horizontal plane, means for measuring the period of oscillation of the filter assembly and for comparing the period measured to the tare period of oscillation, and means for converting the period difference into an indication of the mass added to the filter during the sampling period. In its preferred embodiment the apparatus includes means for assuring that the filter and matter collected thereon have sufficiently low moisture content during the weighing operation.

United States Patent 1 1 Hanson et al.

' n11 3,744,297 [451 July 10,1973

[54] APPARATUS FOR MONITORING SUSPENDED AEROSOLS AND PARTICULATES IN AGAS [75] Inventors: Jack Orvel Hanson, Wayne; Robert Martin Ross,Radnor, both of Pa.

[73] Assignee: General Electric Company, New

York, N.Y.

[22] Filed: Oct. 12, 1971 [21] Appl. No.: 187,986

[52] US. Cl 73/28, 73/432 PS [51] Int. Cl. G0ln 15/00 [58] Field ofSearch 73/28, 324, 61, 67.2, 73/194 B, 432 PS;177/1, 210

[56] References Cited UNITED STATES PATENTS 3,653,253 4/1972 Olin 73/282,508,543 5/1950 Sebok 73/28 X 3,572,098 3/ 1971 Fogwell.... 73/67.2

2,063,775 12/1936 Wilhelmi.. 73/28 3,011,572 12/1961 Bellier 177/210 X7/l970 Ramsay...

6/1971 Banks 73/432 PS Primary Examiner-Richard C. Queisser AssistantExaminer-Stephen A Kreitrnan Attorney-Allen E. Amgott, William G. Beckeret al.

[ 57 ABSTRACT Apparatus for monitoring suspended aerosols and solidparticulates in a gas. The apparatus consists basically of a filter in aspring-mounted holder, means such as a pump for moving gas through thefilter, means for causing the filter assembly to oscillate substantiallyin a horizontal plane, means for measuring the period of oscillation ofthe filter assembly and for comparing the period measured to the tareperiod of oscillation, and

means for converting the period difference'into an indication of themass added to the filter during the sampling period. In its preferredembodiment the apparatus includes means for assuring that the filter andmatter collected thereon have sufficiently low moisture content duringthe weighing operation.

4 Claims, 4 Drawing Figures 70 //0V AC.

APPARATUS FOR MONITORING SUSPENDED AEROSOLS AND PARTICULATES IN A GASBACKGROUND 'OIFWTHE INVENTION The subject invention relates to the fieldof collection of suspended aerosols and particulates in a gas sampleand, in particular, to the monitoring of such aerosols and particulateson an automatic basis.

In recent years added emphasis has been placed on the control ofpollutants, both gaseous and solid, in the air. In order to provideeffective control over pollutants it is usually first necessary toaccurately monitor the presence of such pollutants. Also, in manyindustiral applications it is necessary to monitor the presence andconcentration of particulate matter in a gaseous medium. Generally,monitoring of particulates in a gaseous medium is accomplished by movinga sample of the gas through a stationary filter, removing the filterfrom the apparatus, weighing it and comparing its weight to that of thefilter before it was installed in the apparatus. Very low concentrationsof particulate matter may be measured by causing the particulates in agaseous sample to contact a sampling surface which is then examinedunder a conventional optical or electron microscope. Also, variousoptical techniques, some of which use such sophisticated equipment aslasers, have been developed to monitor certain types of airbornepollutants.

While particulate monitoring is possible with prior art devices,improvements in accuracy, simplicity, automation and cost are stillhighly desirable.

SUMMARY OF THE INVENTION:

Therefore it is an object of the subject invention to provide animproved apparatus for monitoring aerosols and particulates in a gaseousatmosphere;

another object is to provide apparatus for automatically monitoringaerosols and particulates in a gaseous medium on a preselected orperiodic basis; and

yet another object of the subject invention is to provide apparatus formonitoring aerosols and particulates in a gaseous medium.

The abovementioned objects are satisfied in the subject invention byproviding apparatus which is comprised of a filter assembly including afilter and a spring-mounted filter holder, means for moving the gas tobe sampled through the filter, means for oscillating the filter assemblyin a substantially horizontal plane, means for measuring the oscillationperiod of the filter assembly and comparing it to the tare oscillationperiod, and means for converting the difference in oscillation periodsto an indication of the weight gain of the filter. Also it is preferableto provide means for assuring that the moisture content of the filterand the material collected thereon is of a sufficiently low moisturecontent, such as by heating the filter to vaporize moisture thereinprior to weighing it.

The subject matter which is regarded as the present invention isparticularly pointed out and distinctly claimed in the concludingportion of this specification.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention, however, bothas to organization and method of operation, together with furtherobjects and advantages thereof may best be understood by reference tothe following description taken in connection with the accompanyingdrawings in which:

FIG. I is a side sectional view of a monitor in accordance with thesubject invention;

FIG. 2 is a top sectional view of the apparatus shown in FIG. 1 takenalong the line denoted II-II;

FIG. 3 is a simplified process step diagram of a monitoring process inaccordance with the subject invention; and

FIG. 4 is a block diagram of. an embodiment data storage and calculatingportion of the subject invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In FIGS. 1 and 2 are side andtop views, respectively, of a preferred embodiment of monitor 10 inaccordance with the subject invention. Monitor 10 is to be used tocollect and provide an indication of the concentration of suspendedmatter in a gaseous medium. Suspended matter as used in thespecification and claims is intended to include any non-gaseous materialcapable of being collected onto a filter such as suspended aerosols andsolid particulates. While the gaseous medium with which the monitor isto be used is generally air, the apparatus of the subject invention isapplicable for use in other gaseous mediums.

Monitor 10 is basically comprised of a casing 12, a filter assembly 14,conduit means 16, 18 for ducting the gas to be sampled; pumping means 20for moving the gas to be sampled through conduit means l6, 18; means 22for causing oscillation of filter assembly 14; means 24 for sensing theperiod of oscillation of filter assembly 14; electrical means 26connected to sensing means 24 for operating sensed data and computingoutput data therefrom and timing means 28 for controlling the operationof monitor 10.

Filter assembly 14 is comprised of a filter 14a, a holder 14b for thefilter, a cantilever beam Me which is connected to holder 14b to providea spring mounting therefore, and a mounting bracket 14d which is rigidlyconnected to beam Me to provide a rigid support therefor. Filter 14a ispreferably a commercially available high efficiency glass fiber filterbut may be made of any other suitable filter material such as open cellfoam, sintered glass, membrane fibers, organic poly mer, carbon fibers,etc.

The gas conduits include inlet conduit 16 which extends up to anaperture in casing 12 and has its downstream end located adjacent thetop surface of filter 14a; moveable conduitIS which has a gas tight seal30 at the upstream end thereof and a flexible tube 36 which connects thedownstream end of conduit 18 to the input of pumping means 20. ConduitI8 is attached to an actuating linkage 32 which in turn can be actuatedby a seal engaging solenoid 34. A guide 3'7 is disposed about a portionof conduit 18 to permit seal 30 to effectively engage the lower surfaceof filter 14a when solenoid 34 isactuated.

Means 20 for moving the gas through the conduits andfilter mayconveniently be comprised of a vacuum pump 38 and a motor 40. Arestricted orifice 42 is provided in the input to pump 38 to helpmaintain a substantially uniform gas flow rate through the filter andconduits. Control of the flow rate through the monitor can be effectedby controlling the. speed of motor 40. A flow meter (not shown) may beinstalled in an input conduit such as tube 36 and connected to the speedcontrol to effect accurate control of the flow rate through the filter,if desired.

Means 22 for causing filter assembly 14 to oscillate is comprised of anactuating solenoid 23 which is connected to cantilver beam 14c by alinkage 44. Actuating solenoid 23 is preferably disposed in a horizontalplane so as to cause filter assembly 14 to oscillate in a substantiallyhorizontal plane. Solenoid 23 is electrically connected to and itsactuation controlled by timing means 28.

Means 24 for sensing the oscillation frequency of filter assembly 14 iscomprised of a light source 240, a photodiode 24b, and an opticalchopper 45 which is rigidly attached to filter holder 14b. Oscillationof filter assembly 14 causes chopper 45 to periodically attenuate thelight coming from light source 24a to photodiode 24b. The voltage outputof photodiode 24b will therefore vary in amplitude at the same frequencyas the oscillation frequency of the filter assembly 14. The output ofphotodiode 24b is electrically connected to timing means 28 and theelectrical means 26 which provide the necessary computations. Means 26and 28 comprise the mechanical and electrical components used forcontrolling the sequencing of operations of the monitor and forprocessing the information received to provide an output indicative ofthe weight gain of filter 14a which is, of course, due to a collectionof suspended matter on filter 14a. One embodiment of means 26 and 28 isdescribed in detail with respect to FIG. 4.

Where the relative humidity of a gas being collected is greater than 55percent, it has been found that the weight of the collected materialwill be markedly affected (G. P. Tierney and W. D. Conner, l-lygroscopicEffects on Weight Determinations and Particulates Collected on GlassFiber Filters, Amer. Ind. Hyg. Association, July 1967, pp. 363-365).Therefore, the subject invention in its preferred form also providesmeans 46 for lowering the moisture content of the filter material andthe matter collected thereon. in FIG. 1, the removal of moisture isaccomplished by heating the gas to be sampled just prior to its exposureto the filter material by the use of means 46. Means 46 is shown in FIG.1 as being comprised of an enclosure 48 having a through passageway 50in alignment with the input end of conduit 16 and a resistance heater 52located in passageway 50. Of course, other suitable heating means may beused to heat the gas prior to its contact with filter 14 or,alternatively, filter 14 could be heated directly by any conventionalmeans, such as, for example, radiant heater 15. Any other means ofmaintaining the low moisture content, such as the use of desicants, mayalternatively be utilized.

HO. 3 is a block diagram of a preferred process embodiment in accordancewith the subject invention. After a clean filter is inserted in theholder and before the seal engaging solenoid 34 is actuated, solenoid 23is actuated so as to cause filter assembly 14 to oscillate 56 in asubstantially horizontal plane. Oscillation in a horizontal plane ispreferred as it minimizes any effect due to gravity. As assembly 14oscillates, light to photodiode 24b is periodically interrupted byoptical chopper 45 which is connected to the assembly so that anelectrical output from photodiode 24b is produced whose amplitude variesat the same frequency as the frequency of oscillation of the filterassembly. An initial number of cycles are disregarded S8 to allowunwanted transients to damp out. The period of oscillation of filterassembly 14 is then measured 60, for example, by measuring the timenecessary for a preset number of oscillations to transpire. The periodof oscillation is, of course, the reciprocal of the frequency ofoscillation. The data indicative of the measured period of oscillationof the filter assembly with clean filter is then stored 62. This valueis called tare. When the sampling is to begin, pump 38 is actuated andseal 30 of conduit 18 is allowed to engage the underside of filter 14aso that only the gas to be sampled is pulled through filter 14a and thegas sampling conduit system.

The gas will be pulled through the inlet of passageway 50. Heatingelement 52 may be energized during the entire gas sampling, or,alternatively, may be energized no less than five minutes before thepump is to be stopped and the filter assembly weighed. It should benoted that the sampling system is controlled to provide a substantiallyconstant flow rate through the filter which can be preset within a givenrange. When the sampling time is completed, solenoid 34 is actuated soas to disengage seal 30 from filter 14a, and solenoid 23 is actuated soas to cause filter assembly 14 to oscillate 64. As before, an initialnumber of oscillations are premitted to occur 66 before measurement ofthe period of oscillation. The period of oscillation is measured 68 thesame way the tare had been measured and the resulting period ofoscillation data of the filter assembly with the used filter is comparedwith the tare 70. The difference between tare and the measured period ofoscillation is then converted to an output signal 72 which isproportional to the amount of mass added to the filter. The output maybe expressed in any convenient form, such as a digital output 54 (shownin FIG. 1) meter reading, strip chart or magnetic recording.

The mathematical basis for the operation of the w-ieighng system canbest be explained by making a simplifying assumption that the cantileveris massless and that the mass, M. is equal to the mass of the filter andholder plus half of the mass of the actual cantilever. Therefore, theperiod of osciallation T 2 1r VM /K, where K is a form of springconstant which can be obtained experimentally.

If the mass is increased by A M, the period of oscillation becomes T =21r v (M /K) +48 M. The change in period A T= T T, which equals 217/ v?)W V7,).

Consequently A T=27r\/ (M, 7F)( WEE/7, 1 1 M/M Using numerical methodsthis becomes A T= T (AM/2M,,) (l/8) (A M/M 2 H16 (AM/Mhd 0)) a If A M ismuch smaller than M, then A '1 z (T, AM/2M,,)

Thus for practical purposes the change in mass is linearly proportionalto the change in period of oscillation. Experimentally it has been foundthat for an original M. of approximately the accuracy grams the accuracyof the output of the monitor is within 1 percent for a change in mass ofl gram or less.

In FIG. 4 a block diagram of the elctronics of means 26 and 28 which areused to control operation of the monitor, operate on sensed data, andcompute output data is shown.

Standard TTL (transistor-transistor logic) integrated circuits canconventiently be used to implement the logic The analog comparators andpiece parts described can be standard commercially available items. Dualcoil latching reed relays which can be driven directly from the logic,may be used in the relay memory. An example of how the electronics mayoperate is described below:

In the operation of the reset switch, S2 (shown in the standby position)which is located .on a reset relay (not shown), is used to remove thereset signal at the beginning of the measurement sequence. The resetrelay is energized coincidentally with the actuating solenoid 23 and,after a delay of 0.1 second, capacitor C charges analog comparator 74,threshold and the reset signal is removed. Thus, the system is held in astandby mode until all of the high power switching transients haveoccurred. At the end of the sequence, actuating solenoid 23 and resetrelay are de-energized and the system returns to the standby mode.

As discussed above, tare, which is indicative of the weight of theunused (clean) filter must be determined and stored by the electronicsbefore a normal weighing operation can take place. S1 A&B (shown in thenormal position) is manually placed in the load position. This appliespower to the tare relays in the memory and inhibits the 20 bit digitalcomparator 76 output at NAND 5 to preclude any initial tare memorycontents from interrupting the sequence. After the cantilever has beenactivated and system reset removed, the photodiode-analog comparator 78circuit provides a square wave of the same frequency as the cantileveroscillation. This signal drives binary counter 80, used to both discardthe first l6 cantilever cycles and gate the crystal oscillator 82through NAND 3 during the next 32 cycles. At the 16th count, the count16 output goes low and sets the control flip flop 84. The zero output offlip flop 84 triggers one shot 1, resetting the cantilever cycle countback to zero through NAND 1. At the same time, the one output of flipflop 84 enables NAND 3, starting the 20 bit time count accumulater 86(binary counter). Binary counter 80 continues to count to 32 (no signalsare generated by the count 16 output the second time) at which time'thecount 32 goes low. This disables counter 80 at NAND 2 and counter 86 viaNAND 3. The count 32 output also starts binary counter 88. In themeantime, crystal oscillator 82 has been gated into counter 86.Oscillator 82 operates at 320 KHz and is counted down to 16 KHz incounter 90 before it is gated into counter 86. The number of countsaccumulated during the 32 cantilever cycles corresponds to the period ofoscillation of the filter assembly and is therefore indicative of thefilter tare weight. Finally, the load sequence counter (counter 88)generates a load signal (via NAND 8) to the memmy 92 and the contents ofbinary counter 86 are transferred to the tare latching relays in memory92 (only the tare relays are energized by S1). Counter 88 generatesanother signal to turn itself off through NAND 6 and NAND 7 and thesequence is complete.

For a normal measurement, S1 is placed in the normal position, enablingcomparator 76 output at NAND 5 and energizing the D/A relays in memory92. During this sequence, operation of the counter 80, control flipflop84, and load sequence counter 88 are the same. However, as theparticulate weight increases, the cantilever oscillates slower and the32 cycle measurement interval becomes longer. Thus, more 16 KHz countsare accumulated in time counter accumulator 86. The oi'iginal 20 bittare accumulation is fed to comparator 76 by the tare relay contacts(even though the tare relays are not energized, the contacts retain thetare information). When the outputs of the 20 bit accumulation (whichare also fed to digital comparator 76) are equal to the 20 bit tare,digital comparator 76 triggers one shot 2, resetting counter 86 back tozero through NANDs 4 and 5. The additional 16 KHZ count accumulation,corresponding to the particulate weight, is then counted from zero andcan be directly transferred to the D/A (Digital to Analog) converter 94.The first 12 bit outputs of counter 86 correspond to a 1 gram nominalfull scale, so bits 3 through 12 are fed to D/A converter 94 giving a ih milligram output resolution.

Consequently, bits 3-l2 are loaded into the D/A latching relays inmemory 92 at the end of the measurement cycle (only the D/A relays areenergized by S1).

The above-mentioned components and their operation are included only asan example of one way in which the weight gain due to collection ofsuspended matter on the filter can be measured.

It is obvious that many modifications may be made to the apparatusdisclosed herein within the true scope and spirit of the invention. Forexample, the filter and filter holder can be spring mounted in any oneof the number of configurations. As examples, instead of using acantilever beam, the filter holder can be mounted in the middle of aspring-type beam fixed at both ends or the filter holder and filter canbe attached to a rigid rod pivoted at one end with one or more coilsprings horizontally positioned with one end fixed and the other endfixably attached to the rod or filter holder. Also, initiation ofoscillation may be effected by any one of a number of means other thanthe electrical solenoid arrangement discussed above including a springloaded mechanical actuator, a fluid actuator or any type of electricalor magnetic actuator. While it is preferred to have the filter assemblyoscillate freely after initial actuation, as is well known in the artcontinuous actuation at the filter assemblys natural frequency ofoscillation can be achieved using state of the art electricalcomponents. Also, the period of oscillation of the filter assembly canbe sensed by the use of other optical means, electrical, mechanical, orfluid apparatus. For example, movement of a portion of the filterassembly in a magnetic field will cause an electrical signal to begenerated whose frequency is the frequency of oscillation of the filterassembly. Another alternative is to allow a portion of the filterassembly to interrupt a fluid stream from a supply nozzle to a receiverpassageway when the filter assembly oscillates so as to produce a fluidoutput signal having a pulsation frequency equal to that of thefrequency of oscillation. If a pressure transducer is used in thereceiver passageway the fluid pulsations can be converted to anelectrical signal whose frequency is the same as the frequency ofoscillation of the filter assembly. Of course, many other designs ofelectrical circuits or mechanical apparatus may be used to compare theperiod of oscillation of the filter assembly after sampling occurs withtare. While this specification discloses that the period of oscillationcan be obtained by measuring the time necessary for a preset number ofoscillations to occur, it is obvious that the frequency of oscillationcan be obtained by counting the number of cycles which occur within apreset time interval.

Thus, it is intended that the subject invention be limited only by thefollowing claims.

What is claimed and desired to be secured by Letters Patent of theUnited States is:

1. Apparatus for monitoring particulates in a gas comprised of:

a. a filter rigidly attached to a spring mounted holder assembly; b.means for moving the gas to be sampled through said filter; 0. means forcausing said holder assembly to oscillate substantially in a horizontalplane; and d. means for measuring the period of oscillation of saidholder assembly and filter. 2. Apparatus as in claim 1 further includingmeans for computing the weight gain of the filter.

3.' Apparatus as in claim 1 further including means for heating saidfilter.

4. Apparatus as in claim 3 wherein said filter heating means includesmeans for heating the gas prior to its contact with said filter.

1. Apparatus for monitoring particulates in a gas comprised of: a. a filter rigidly attached to a spring mounted holder assembly; b. means for moving the gas to be sampled through said filter; c. means for causing said holder assembly to oscillate substantially in a horizontal plane; and d. means for measuring the period of oscillation of said holder assembly and filter.
 2. Apparatus as in claim 1 further including means for computing the weight gain of the filter.
 3. Apparatus as in claim 1 further including means for heating said filter.
 4. Apparatus as in claim 3 wherein said filter heating means includes means for heating the gas prior to its contact with said filter. 